1. Field of the Invention
The present invention relates to an immersion nozzle for introducing molten steel from a tundish to a mold for continuous casting of steel, and more particularly to a structure of the immersion nozzle.
2. Description of the Prior Art
Deposit of oxide inclusions on the inwall of an immersion nozzle for continuous casting of steel increases as time goes by. This deposit restricts casting time of molten steel, builds deoxidized products of a few microns contained in molten steel and induces defects in steel products. An increase of the defect ratio of the products attributable to mold powder has also been ascertained in connection with the recent increase of continuous casting speed. This increase of the defect ratio is closely related with up and down movements of the surface level of the molten steel in the mold. The excess movement of the surface level of the molten steel over a certain level gives rise to the defects attributable to the mold powder. In the increase of the continuous casting speed, the up-and-down movements of the surface level of the molten steel are too difficult to be controlled in the range of the optimum levels and their results are remarkably reflected as the defects of the products. Properly speaking, the forms of the immersion nozzle need to be selected individually and elaborately depending on continuous casting speed and size of the slabs, because the movements of the surface level of the molten steel in the mold are determined by the flow speed and the flow direction of the molten steel poured from the immersion nozzle into the mold. However, the proceeding of the increase of the inclusions deposited on the inwall of the immersion nozzle varies the flow speed and the flow direction of the molten steel poured from the immersion nozzle into the mold as time goes by and often causes surface defects of the slabs attributable to the mold powder.
FIGS. 1(a)-1(c) show sectional views illustrating schematically a prior art immersion nozzle. FIG. 1(a) is a sectional plan view of an immersion nozzle body taken on line 2--2 of FIG. 1(b), passing through the centers of exit ports 12a and 12b. FIG. 1(b) is a vertical sectional view of the immersion nozzle body taken on line 3--3 of FIG. 1(a). FIG. 1(c) is a vertical sectional view of the immersion nozzle body taken on line 4--4 of FIG. 1(a). Prior art immersion nozzle body 11 has bore 13 (for passing the molten steel) inside the immersion nozzle and two exit ports 12a and 12b facing each other in the lower portion. The cross-sectional area of bore 13 is the same over the entire length of the nozzle. A horizontal inner length (i.e., diameter) of exit ports 12a and 12b are substantially the same as the diameter of bore 13. Alumina-graphite or zirconium is used for immersion nozzle body 11. Reference numeral 14 schematically shows inclusion deposited on and built up on the inwall of the immersion nozzle. The prior art immersion nozzle has difficulties in that the inclusion deposit on the nozzle inwall and the surface defects attributable to the mold powder occur.